A couple with a small baby asked if they could be moved together – umm... you didn't ask the agents why?
The airlines tend to pass the buck on this.  Even if you reserve seats together and call and link PNRs, sometimes you get split up.  On the phone they tell you they can sort it out at the airport.  The gate agents say the FAs will take care of it.  The FAs suggest that you canvass your fellow pax and see if someone will switch.  (One possible valid reason for the latter, suggested on The Other Board, is that if the FA asks someone to switch they may demand vouchers or some form of compensation.)   DL pulled this with Ron and me (although they did respond to my begging and gave Ron an aisle seat on a TATL).  We were in rows across from each other, me in Window, Ron in Aisle.  Ron was flying with 2 teenage brothers who were seated together; I was seated with an elderly couple and the man had vision problems and clearly needed his wife nearby to help him with things.  We dealt with it, but clearly there are times when not being seated together is a bigger problem.